It is easy to buy more than is needed initially. It's probably better to buy things you find you need as you go, than to try to imagine every possible eventuality at the start.
I'd suggest the first camping "trip" or two be in your own driveway, if possible, for a couple reasons. First, you'll have a chance to figure out or remember how all the systems work on the RV with less stress. Second, if you find you need something that you don't have, it's ready at hand, and you can make a note to buy one for the trailer if applicable. This is especially useful for kitchen supplies and paraphanalia, though it applies everywhere.
Do consider how long you'll likely be keeping the RV when looking at floorplans. Many of the bunks in bunkhouses I've seen are none too long, and may be fine for an 8 year old but could become pretty nearly useless in five years when the kid has become a lanky 13 year old. Also be thinking or even miming what you'd be doing to go about normal daily household tasks: can you turn around and reach the middle of your back in the shower? can you cut up veggies for a salad in the kitchen while cooking pasta? is there room for a trash can in a handy location? would there be room to do homeschool work and plan the next leg of travel at the same time?